Conventional iron-chrome high temperature water gas shift catalyst typically operate at temperatures from 350° C. to 450° C. and have been proven to be active and stable. However, there are unique H2 production designs being developed where active, stable and selective water gas shift catalysts are required to operate at much higher temperatures. These temperatures can occur, for example, in reforming systems that have been developed for on-site hydrogen production for industrial and high temperature fuel cell applications. In these situations the temperature for the first water gas shift stage can be as high as 900° C., thereby matching the reforming catalyst exit temperature and/or matching the temperature of the fuel cell stack. At these temperatures conventional iron-chrome catalysts degrade due to physical loss of strength. When operated at these temperatures, these catalysts also are prone to make heavy hydrocarbons via a Fischer-Tropsch reaction.
On-site hydrogen production units and high temperature fuel cell power plants that utilize a fuel cell stack for producing electricity from a hydrocarbon fuel are known. One example of these power plants is a molten carbonate or a solid oxide fuel cell where the operating temperatures are from 600-1000 C. With these systems, matching the water gas shift catalyst operating temperature to the reforming catalyst or fuel cell operating temperatures is beneficial as the system is simplified by elimination of heat exchangers and other associated equipment and controls.
The hydrocarbon fuel for such fuel cell stacks can be derived from a number of conventional fuel sources, with preferred fuel sources including, but not limited to, natural gas, propane and LPG.
In order for the hydrocarbon fuel to be useful in the fuel cell stack, it must first be converted to a hydrogen rich fuel stream. After desulfurization, the hydrocarbon fuel stream typically flows through a reformer, wherein the fuel stream is converted into a hydrogen rich fuel stream at temperatures up to 900° C. This converted fuel stream contains primarily hydrogen, carbon dioxide, water and carbon monoxide. The quantity of carbon monoxide can be fairly high, up to 15% or so.
Anode electrodes, which form part of the fuel cell stack, are adversely affected by high levels of carbon monoxide. Accordingly, it is necessary to reduce the quantity of carbon monoxide in the fuel stream prior to passing it to the fuel cell stack. Reduction of the quantity of carbon monoxide is typically performed by passing the fuel stream through a water gas shift converter. In addition to reducing the quantity of carbon monoxide in the fuel stream, such water gas shift converters also increase the quantity of hydrogen in the fuel stream.
Water gas shift reactors are well known and typically contain an inlet for introducing the fuel stream containing carbon monoxide into a reaction chamber, a down stream outlet, and a catalytic reaction chamber, which is located between the inlet and outlet. The catalytic reaction chamber typically contains catalytic material for converting at least a portion of the carbon monoxide and water in the fuel stream into carbon dioxide and hydrogen. The water gas shift reaction is an exothermic reaction represented by the following formula:CO+H2O⇄CO2+H2.
Water gas shift reactions are usually carried out in two stages: a high temperature stage, at temperatures typically from about 350° C. to 450° C. and a low temperature stage at temperatures typically from 180° C. to 240° C. While the lower temperature reactions favor more complete CO conversion, the higher temperature reactions allow recovery of the heat of reaction at a sufficient temperature level to generate high pressure steam.
Because of various unique operating conditions, as discussed above, water gas shift reactions sometimes occur at temperatures above 550° C. and even as high as 900° C. or so. However, at these temperatures, the excess production of methane and the formation of higher hydrocarbons, generally by a Fischer Tropsch reaction, by the water gas shift catalyst are significant issues.
In addition, conventional water gas shift catalysts are not able to physically withstand these higher operating temperatures. These high temperatures are experienced in reformer designs where the high temperature reforming steps are thermally integrated in so-called heat exchanger reactors. Such high temperatures also occur when the water gas shift catalysts are thermally integrated with high temperature fuel cells.
There are a number of water gas shift catalysts that are known in the art. For instance, known water gas shift catalysts may contain chromium, copper or precious metals, preferably platinum, palladium, rhodium or ruthenium, as the active component, deposited on a support. In one preferred embodiment Pt and/or Ru and/or Pd and/or Rh are deposited on a conventional support. Such precious metal based water gas shift catalyst generally operate at 300° C. to 400° C. Conventional iron-chrome water gas shift catalysts are generally operated at temperatures from 350° C. to 450° C.
Notwithstanding the existence of various compositions for catalysts for use in water gas shift converters, there still a need for improvements in the performance of these water gas shift catalysts, particularly in activity, stability and limitation on methanation and higher hydrocarbon production at high temperatures above 550° C. up to 900° C. or so. Further, at these high temperatures, conventional water gas shift catalysts physically degrade.
In addition, when conventional water gas catalysts are modified to prevent the formation of higher molecular weight hydrocarbons and by-products, activity of the catalysts is frequently reduced.
Many precious metal water gas shift catalysts, particularly platinum, rhodium, palladium and/or ruthenium-based water gas shift catalysts, cause methanation of CO and/or CO2 as a side reaction when operated at temperatures above about 325° C. A large quantity of the hydrogen present in the feed stream can be consumed by these methanation reactions and thereby, reduce the overall yield of hydrogen. Further, methanation of carbon oxides is accompanied by a strong exothermic reaction which causes a rapid temperature increase, thereby making control of the reaction difficult and reducing the stability of the catalyst.
For purposes of this disclosure “high or higher temperature” water gas shift reactions are those that occur at a temperature greater than about 450° C., generally greater than 550° C. and up to as high as about 900° C., or so.
Accordingly, it is one object of one embodiment of the invention to provide an improved water gas shift catalyst that retains activity to achieve equilibrium, particularly at high temperatures.
It is a further object of one embodiment of the invention to provide an improved water gas shift catalyst for use at high temperatures that does not result in any substantial methanation reactions or the production of substantial quantities of higher hydrocarbons.
It is the further object of one embodiment of the invention to provide an improved water gas shift catalyst with increased stability over the lifetime of the catalyst.
It is further object of one embodiment of the invention to provide a process for the preparation of these improved water gas shift catalysts.
It is understood that the forgoing detailed description is explanatory only and not restrictive of the invention.